The IMPATT diode is a 2-terminal device with typical applications in the area of radio frequency (RF) power generation and amplification. Compared to a 3-terminal device approach, the IMPATT diode can be fabricated to have very small resistive loss and parasitic capacitance. Therefore, the IMPATT diode can generate high RF power at high frequency, making it especially useful for the Terahertz (>300 GHz) applications.
A conventional n-type IMPATT diode, as shown in FIG. 1 consists of three distinct regions, a heavily doped P++ region 101 for avalanche breakdown, a lightly doped N region 102 for charge drift, and a heavily doped N++ region 103 for charge collection. When the diode is reverse biased, the free electrons inside the N region are depleted from the device, creating a peak electrical field at the P++/N junction. When the reverse DC bias increases, the peak electrical field increases, until one of two breakdown processes occurs. In one process, the field may be so high that it exerts sufficient force on a covalently bound electron to free it. This creates two carriers, a hole and an electron to contribute to the current. This breakdown is called Zener breakdown or tunneling breakdown. In the second breakdown process, the residual free carriers are able to gain enough energy from the electrical field and break covalent bond in the lattice. This process is called avalanche breakdown, and every carrier interacting with the lattice as described above creates two additional carriers. All three carriers can then participate in further avalanching collisions, leading to a sudden multiplication of carriers in the space-charge region when the maximum filed becomes large enough to cause avalanche,
Once the carriers are created by breakdown in the high field region, the holes will flow out of the device from the top Ohmic contact, resulting in DC current. The electrons will travel across the N region (drift region) 102 and flow out of the device through the bottom Ohmic contact. With proper designed doping profile, the electrical field in the N region 102 will be high enough that all the electrons will move at their saturation velocity vsat. Since the thickness of N region is nonzero, the electrons take finite time, called transit-time, to flow out of the device. Under alternating current (AC) condition, the diode AC current, coming from the moving electrons within the device, can lag behind the AC voltage applied on the diode, resulting in phase delay between AC current and AC voltage. In the IMPATT diode, the thickness of the N region (drift region) is designed properly to create 180 degree phase delay, therefore the diode shows negative resistance. Once such diode is connected with a resonant circuit, the diode negative resistance can create oscillation and generate RF power.
Typically, a silicon IMPATT diode is fabricated vertically in mesa structure, as in Henry's U.S. Pat. No. 3,896,478. Similar structures are also disclosed in Henry's U.S. Pat. No. 3,649,386 and Lee's U.S. Pat. Nos. 4,030,943 and 4,064,620. Such mesa structures are still widely used in recent works. Bayraktaroglu, from TI (Texas Instruments), disclosed a slightly different approach, as in U.S. Pat. No. 4,596,070, to fabricate the IMPATT diode where polyimide is used to isolate different active diodes.
Two major source of series parasitic resistance must be minimized, and these resistances are: 1) contact resistance at the substrate contact metal interface; 2) series resistance of the substrate modified by skin effect. Contact resistance is reduced by maximizing the effective doping level in the substrate at the contact surface either by maintaining a high level of substrate doping or by contact alloying. Minimizing substrate resistivity also reduces the skin effect contribution to the series resistance. To minimize series resistance, the diode substrate is thinned to micrometer range.
The discrete mesa shape IMPATT diode in FIG. 1 becomes difficult to adopt at Terahertz regime. In this frequency range, an optimized diode should have a diode diameter smaller than 5 μm. It is challenge to fabricate such small diode with thinned substrate, and still be able to assemble the package with desired electrical property, good reproducibility, and long term reliability. Therefore an integrated IMPATT diode fabrication technique is needed to solve the above challenges.